Disclosed methods and apparatus generally relate to inductive remote plasma sources for applications such as treating surfaces, device fabrication, treatment of materials and products, lighting, and others. Furthermore, inductive plasmas disclosed herein can be useful sources of chemically active species, charged particles such as ions or/and electrons, charged and/or neutral species in excited states, and sources of coherent and/or incoherent ultraviolet, visible, and/or infrared radiation. Various embodiments are useful for processing substrates downstream of the plasma source, cleaning plasma processing chambers, manufacturing semiconductor devices, illumination, laser excitation, and others.
The terms “remote plasma source” and “remote plasma processing” generally relate to plasma generation apparatus and plasma treatment methods (e.g. device processing, surface treatment, plasma cleaning) in which plasma generation and treatment/processing with entities are effectuated in distinct spatially segregated regions (e.g. a treatment/processing position is separated from the plasma generation region).
Inductively coupled plasmas (ICP) for generating active species for cleaning chambers and remote plasma processing have been maintained using apparatus comprising an inductive applicator operable to couple high frequency power into a plasma within a chamber (the plasma source chamber). The applicator can be external to the chamber and isolated from the plasma environment.
In the prior art, a single external helical coil wound on a tubular quartz or ceramic chamber has been useful to sustain an inductive plasma discharges to generate active species in applications such as remote plasma cleaning, surface treatment, resist ashing, etching, chemical vapor deposition, gas discharge lighting, and gas lasers. Depending on the application, active plasma species useful for processing can be neutral radicals, excited states of various atomic and/or molecular entities, and/or charged particles such as electrons and ions.
In a number of applications remote plasma processing has been preferred. A remote plasma apparatus has spatially segregated plasma generation and treatment regions. A plasma can be generated in a plasma source chamber, and species emanating from the plasma source. and/or secondary species arising from the emanated species, can be transported downstream to a distinct (“remote”) region for processing. In this manner, exposure to unwanted and/or harmful agents inherent in a plasma generation region, such as current, heat, charging current, short wavelength radiation (ultraviolet, x-ray, etc.) and others can be selectively attenuated, or even eliminated from the processing region. Remote processing can be performed in a discrete chamber, and/or in a conduit downstream of the plasma generation chamber, depending on the application.
Various process feed gases can be admitted to the plasma generation (source) chamber to produce active species. For example, a feed can comprise elemental gases such as helium, argon, chlorine, bromine, hydrogen, oxygen, nitrogen and others, and compound molecular gases such as fluorocarbons, chlorocarbons, silanes, water vapor, ammonia, and others. Furthermore, additional feed gas can be admitted downstream from a remote plasma source to produce secondary species arising from reaction between the additional feed and species emanating from the remote plasma. In embodiments, a gas can be evacuated from a chamber with pumping means such as a roots blower, a mechanical pump, a turbomolecular pump, and/or others, in single or in combination.
Various prior art ICP remote plasma sources can be understood with respect to in FIGS. 1A, 1B, 2A, and 2B. FIGS. 1A-1B show a general prior art inductively coupled remote plasma source embodiment. The embodiment comprises a cylindrical tube (“chamber”) in which an inductively coupled plasma can be sustained. A helical coil 130 wound around the tube can be energized with a radiofrequency power source (not shown). Typically, high frequency voltage from the power source is applied to the coil and drives current through the coil. The high frequency current flow can induce high frequency magnetic flux lines in the tube, generally directed along its axis (axially). This axial time-dependent flux can actuate a circumferential radiofrequency plasma discharge current circulating in a torus 115 within the tube (FIGS. 1A-1B).
The straight passage of a prior art tubular inductively coupled remote plasma source such as shown in FIGS. 1A-1B, presents a relatively low resistance to gas flow. However, the active plasma channel in this configuration is often susceptible to radial and axial plasma constriction, particularly at relatively high pressure and/or power levels. Even in normal operation, a majority of active plasma current is contained within a toroidal donut-like region 115 close to tube wall 110, as shown in FIGS. 1A-1B. Furthermore, the axial extent of this donut-shaped region (dimension along the direction of the tube) is generally limited to the order of a tube radius, is relatively independent of the coil length. Therefore, increasing the amount of active plasma volume in this configuration has been problematic.
Another inductively coupled remote plasma source embodiment described in U.S. Pat. No. 6,150,628 by Smith et al. can be understood with respect to FIGS. 2A and 2B. Exemplary embodiments with respect to FIGS. 2A and 2B comprise a chamber 204 having at least two parallel connected tubular flow passages 205, 206 (vertical left and right channels shown in FIG. 2A). Ferromagnetic elements 230, 231 encircle each of the respective parallel flow passages. As can be seen with respect to the transverse cross section in FIG. 2B, coils 242, 244 can be wound on the respective ferromagnetic elements 230 and 231 (the coils are not shown in the simplified vertical cross section FIG. 2A). Each coil is operable to receive high frequency current from a power source to induce a circulating high frequency magnetic flux 252, 251 in the respective core (230, 231). The magnetic flux circulation around each flow passage in turn induces an electromotive force (EMF) directed along the axis of a respective passage.
The direction of induced EMF depends on the sense of current flow in the coil and resultant sense of magnetic flux circulation (e.g. counterclockwise flux circulation 251 provides positive EMF emerging from the plane of the figure and clockwise circulation provides EMF having the opposite sense). The relative phase of the coil currents is selected to provide an EMF in each flow passages 180 degrees out of phase with respect to other. Accordingly, the EMF's are in a serial relationship, and can drive active plasma current 215 in a closed loop through the flow passages.
Embodiments with respect to FIGS. 2A-2B are often susceptible to radial plasma constriction. In radial plasma constriction, the width of the active current carrying plasma path shrinks to a narrow, relatively small fraction of the inner tube (chamber) diameter. Correspondingly, substantial amounts of feed gas can bypass the active region and receive little or no excitation. It has been found that increasing the fluid passage diameter (e.g. using a larger tube/chamber cross section) has relatively little effect on the active plasma radius (e.g. the portion of the tube cross section conducting induced current which activates the plasma generally shows little change). Accordingly, it has been difficult to increase the scale of active species production in these sources.
Remote inductively coupled plasma sources such as shown in FIGS. 1A, 1B, 2A and 2B, are often operated in a relatively high range of gas pressure (0.1-10 Torr or more) and relatively high radiofrequency power (1 KW and above). Under these conditions, a steady state unconstricted plasma mode can be unstable with respect to constriction.
As can be seen, there is a longstanding need for inductively remote plasma sources that can be scalable to provide relatively larger and stable active plasma volumes and/or higher power levels, and have simple construction.